This invention relates to a frost sensor for use in defrost controls for refrigeration and a system that incorporates the frost sensor. In particular, the invention relates to an apparatus and method for sensing the buildup of insulating accumulations or materials (e.g., ice or dirt) on the evaporative heat exchanger of a refrigeration system and for controlling the operation of the system.
The evaporative coils of commercial and industrial refrigeration systems form frost because the evaporative coil surface is below freezing and warmer moist air is blown across the coil in order to cool the refrigerated space. This frost deposition accumulates over time to form ice deposits that reduce the efficiency of the refrigeration system. The efficiency is reduced because the ice acts as a thermal insulator of the cooling coil and also blocks airflow over the cooling fins of the coil. It is therefore necessary to defrost the evaporative coils periodically in order to maintain cooling capacity. Current defrost controls are predominantly time clocks which are typically set to defrost too often and for too long a period.
Significant power savings can be realized by defrosting the heat-exchanger in refrigeration systems only when necessary, and only for as long as necessary. On the order of 5 percent to 12 percent of the total power consumed in refrigeration can be saved by the proper implementation of a demand-defrost system. For a commercial system to be successful, it must: (1) use a rugged sensor (having a longer life than the refrigeration unit, with zero maintenance), (2) have universal application to different heat-exchangers, (3) add no complexity to system operation, and (4) be low-cost in terms of hardware and installation. A variety of types of demand-defrost systems have been proposed.
Optically-based sensors have been installed to detect frost buildup. When frost reaches a preset thickness, a defrost cycle is initiated. Several sensors of this type have been marketed with limited success, primarily because the sensors trigger unnecessary defrosts due to dirty environments and require intensive maintenance. Because sensors have to be located at the display-case evaporator, replacement of a failed sensor is very difficult, often requiring removal of refrigerated product from the case. For this reason, failed sensors are generally replaced with standard timers.
A temperature-sensor-based demand-defrost controller was developed and introduced by Honeywell. The controller employed two temperature sensors located at the inlet and outlet of the display case evaporator. Defrost was initiated when the temperature difference between the two sensors reached a defined setpoint. This controller did not gain wide acceptance because it was prone to triggering defrosts unnecessarily.
A system that was developed but was never made commercially available used the temperature difference between the inlet to the heat-exchanger fan and the air temperature after blowing over the heat-exchanger. Application of this system was limited to systems with specific hardware configurations. Identifying the actual trigger point for defrosting for most systems added too much complexity for the average operator. In addition, the formation of ice on the heat-exchanger blocked much of the air flow and thereby produced a lower flow of air that still had a large enough temperature drop so that defrost was not initiated even after the heat-exchanger frosted.
Pressure-sensor technology measures the change in refrigerant pressure through the evaporator as frost is deposited on the surface and can signal when defrost is required. The magnitude of this pressure difference is very small (a fraction of an inch of water) which is difficult to measure accurately. Moreover, the cost of accurate pressure sensors is prohibitively high, which makes them unattractive for defrost control.
A number of systems that rely on some type of adaptive control have been proposed, with a moderate degree of technical success. These systems were commercial failures because of the complexity of troubleshooting. Since the triggering is not based on a simple, single measurement, the operators cannot easily identify the problem. In virtually all cases, the system is immediately bypassed, reverting back to timing clocks. As a practical matter, once a system has been bypassed, it is seldom repaired.
The background art is characterized by U.S. Pat. Nos. 3,525,648; 3,854,915; 3,945,217; 4,142,374; 4,156,350; 4,173,871; 4,305,259; 4,329,682; 4,344,294; 4,345,441; 4,347,709; 4,348,870; 4,409,795; 4,481,785; 4,528,821; 4,530,218; 4,532,806; 4,608,832; 4,671,072; 4,882,908; 4,993,233; 5,051,645; 5,319,943; 5,345,775; 5,493,867; 5,522,232; 5,692,385; 6,038,872; and 6,092,925; the disclosures of which patents are incorporated by reference as if fully set forth herein.
Poppendiek in U.S. Pat. No. 3,525,648 discloses a thermoelectric heat flow responsive device. An embodiment of this device is incorporated into the invention of U.S. Pat. No. 4,608,832, discussed below. This device is limited in that it appears to be relatively thick, possibly over ⅙ inch in thickness. Moreover, the device has a substantial resistance to heat flow due to the relatively high number of interfaces between the layers that make up the device and due to the thickness of the device. This suggests that a temperature drop of several degrees would occur across the device. This substantial resistance limits the applications of the device to other than frost sensing in a refrigeration system. If the air-exposed side of a heat flux sensor is several degrees warmer than the adjacent fin surface, ice will preferentially form on the fin and not on the heat flux sensor, rendering a defrost control system that incorporates such a heat flux sensor inoperative.
Schulze-Berge et al. in U.S. Pat. No. 3,854,915 disclose a demand defrost system. This invention is limited in that a periodic switch device is required to initiate defrost cycles in response to relative humidity values ambient to a refrigeration system.
Barshark in U.S. Pat. No. 3,945,217 discloses a refrigeration system defrost control. This invention is limited in that it includes a sensor that exhibits a change in resistance as a function of the amount of moisture it absorbs. When the sensor has absorbed a certain amount of water, a defrost cycle is initiated. The sensor must be dried during the defrost cycle.
Ansted et al. in U.S. Pat. No. 4,142,374 disclose a demand defrost time clock control circuit. This invention is limited in that the construction and method of operation of its frost sensor appears to be a conventional optical frost sensor, preferably produced by Altech, Inc.
Elliott et al. in U.S. Pat. No. 4,156,350 disclose a demand defrost control system and method. This invention is limited in that it bases the interval between future defrosting operations on the time required for the defrost heater to raise the evaporator temperature to a predetermined temperature during a previous defrosting operation.
Brooks in U.S. Pat. No. 4,173,871 discloses a demand defrost control system and method. This invention is limited in that it bases the interval between future defrosting operations on the time required for the defrost heater to raise the evaporator temperature to a predetermined temperature during a previous defrosting operation.
Jaeschke in U.S. Pat. No. 4,305,259 discloses a frost sensor employing a self-heating thermistor as a sensor element. This invention is limited in that current must be applied to the thermistor to cause it to increase in temperature during a frost measurement.
Baker in U.S. Pat. No. 4,329,682 discloses a method and apparatus for providing a warning of icing conditions in an aircraft air conditioning system. This invention is limited in that a thermo-electric heat pump is used to maintain a constant difference between the temperature of a cold surface in an air stream and temperature of the air stream.
Gelbard in U.S. Pat. No. 4,344,294 discloses a thermal delay demand defrost system. This invention is limited in that it requires that sensing of the temperature of the refrigerated air be used to alter the duration of time between defrost cycles.
Hansen in U.S. Pat. No. 4,345,441 discloses a defrost control apparatus for the evaporator of a refrigerator. This invention is limited in that it requires that a temperature sensor be mounted a predetermined distance from a surface of the evaporator. Defrosting occurs when the sensor temperature falls below a reference temperature.
Wu et al. in U.S. Pat. No. 4,347,709 disclose a demand defrost sensor. This invention is limited in that it requires that a capacitive sensor plate be installed next to the evaporator surface and that a noise-immune phase detector be used to detect a phase shift caused by frost buildup.
Stein et al. in U.S. Pat. No. 4,348,870 disclose a temperature probe for an air conditioning device. The device is limited in that it includes a freeze-up thermistor that is used to sense the temperature within an evaporator tube array. The sensor produces a signal that is indicative of freeze-up or insipient freeze-up, which signal is used to terminate the operation of the compressor until a safe temperature is sensed.
Krueger in U.S. Pat. No. 4,409,795 discloses a demand defrost system. This invention is limited in that a photocell is required to sense the build-up of frost in a heat exchanger.
Tershak et al. in U.S. Pat. Nos. 4,481,785 and 4,528,821 disclose an adaptive defrost control system for a refrigerator. This invention is limited in that it controls the length of the interval between defrost operations in accordance with the number and duration of compartment door openings, the duration of a previous defrost operation as corrected by the temperature of the evaporator prior to defrost and the length of time the compressor has been energized.
Janke et al. in U.S. Pat. No. 4,530,218 disclose a refrigeration apparatus defrost control. This invention is limited in that it requires the use of a conventional frost sensor which may be an optical sensor, a pressure sensor or an acoustical sensor.
Bruchmuller in U.S. Pat, No. 4,532,806 discloses a sensor for monitoring the deposition of frozen fog and/or ice on a surface. This invention is limited in that it includes a vibration transmitting membrane and a vibration receiving membrane. When frost is deposited on the membranes, a damping effect takes place.
Sabin et al. in U.S. Pat. No. 4,608,832 disclose means and techniques useful for detecting frost on a fin of an evaporator. This invention is limited in that it is configured to measure the temperature gradient from one point on the fin to another point on the fin using a heat flux sensor. The technique is unreliable because it requires knowing where and how to locate the sensor on the fin, which varies from one installation to the next. While the temperature gradient on a fin is a function of frost buildup, the function is different for each installation. A critical limitation of this invention is that it does not provide structure that is capable of measuring the heat flux from the fin to the air. One embodiment of this invention includes the thermoelectric heat flow responsive device of U.S. Pat. No. 3,525,648, which device has the limitations described above.
Starck et al. in U.S. Pat. No. 4,671,072 disclose a sensor for detecting frost deposits. This invention is limited in that it relies on the use of a heat source and a heat sensor. When sufficient frost forms around the sensor, more heat is conducted from the heater to the sensor, which causes the temperature sensed by the sensor to increase.
White in U.S. Pat. No. 4,882,908 discloses a demand defrost control method and apparatus. This invention is limited in that it requires that a controller check outdoor and coil temperature to determine if a defrost cycle should be initiated.
Borton et al. in U.S. Pat. No. 4,993,233 disclose a demand defrost controller for refrigerated display cases. This invention is limited in that it requires temperature measurements taken at the outlet of the discharge air curtain and the inlet of the air return to determine the need for defrost.
Brace et al. in U.S. Pat. No. 5,051,645 disclose a frost sensor that incorporates an acoustic wave water phase change sensor. This device is limited in that an acoustic wave water phase change sensor is required.
Bahel et al. in U.S. Pat. No. 5,319,943 disclose a defrost control system for a heat pump. This invention is limited in that it requires measurement of the difference between the outdoor air temperature and the temperature of the outdoor coil.
Ridenour in U.S. Pat. No. 5,345,775 discloses a frost detection assembly for a refrigeration system. This invention is limited in that it requires the use of two thermistors, one touching the fin and one located about {fraction (1/16)} inch away. When the depth of ice accumulation on the fin reaches {fraction (1/16)} inch, the temperatures sensed by each of the thermistors become similar. A comparison of the temperatures is used to signal the initiation of a defrost cycle.
Szynal et al. in U.S. Pat. No. 5,493,867 disclose a fuzzy logic adaptive defrost control. This invention is limited in that it requires determination of the cumulative and continuous runs times of the compressor and defrost heater.
Nojiri in U.S. Pat. Nos. 5,522,232, 6,038,872 and 6,092,925 discloses a frost detection device. This invention is limited in that it requires the use of two temperature sensors, one of which has a cap and is a reference sensor and the other of which is in contact with air through small slits. When ice forms over the slits, the differential temperature between the two sensors approaches zero, and a defrost cycle is initiated. The invention also requires the cycling of the refrigerator""s compressor to accentuate the temperature inertia differences.
Hollenbeck et al. in U.S. Pat. No. 5,692,385 disclose a system and method for initiating a defrost cycle. The invention is limited in that sensing of the speed or torque of the motor that drives the fan that moves air through the evaporator is required.
The background art is also characterized by non-patent publications. None of these publications teach the subject invention, but many of them reveal the need for more efficient defrost control.
Eckman, R. L. in xe2x80x9cHeat Pump Defrost Controls: A Review of Past, Present and Future Technologyxe2x80x9d presented at the 1987 Winter Meeting of ASHRAE and published in the ASHRAE Transactions: Technical and Symposium Papers for that conference, (1987), pp. 1152-1156, describes the advantages and disadvantages of a variety of technologies for defrost control. The approach taken by the inventors of the subject invention is not mentioned.
Heinzen, R. A. in xe2x80x9cHow Adaptive Defrost Maintains Refrigeration System Efficiencyxe2x80x9d in Australian Refrigeration, Air Conditioning and Heating, (April 1988), pp. 12-16, 42-4, describes adaptive defrost control. The xe2x80x9cKxe2x80x9d variable as used in this reference represents a different variable from the xe2x80x9cKxe2x80x9d variable used herein.
Paone, N. et al. in xe2x80x9cFiber-optic ice sensors for refrigeratorsxe2x80x9d in SPIE Fiber-Optic Sensors: Engineering and Applications, (March, 1991), pp. 129-130, 1511, describes a defrost control based on fiber-optic technology.
Energetics, Inc. in Refrigeration Systems Program Summary, DOE/CH/10093-120, (December, 1991), pp. 1-8, U.S. Department of Energy, Washington, D.C., describes the goals for the program, which include encouragement of the development of new energy-efficient refrigeration systems.
Borton, D. N. et al. in Development of a Demand Defrost Controller, (October, 1993), pp. i through 4-7, New York State Energy Research and Development Authority, Albany, N.Y., describes demand defrost control based on the temperature difference between the discharge and return of the display case air curtain in combination with several time settings.
Westphalen D. et al. in Energy Savings Potentialfor Commercial Refrigeration Equipmentxe2x80x94Final Report, (June, 1996), pp. i through 3xe2x80x943, U.S. Department of Energy, Office of Building Technologies, Washington, D.C., describes the energy-saving potential of a number of technologies, including demand defrost control.
The Office of Industrial Technologies, U.S. Department of Energy, in Energy Saving Intelligent Controller for Refrigeration (November, 2000), pp. 1-2, U.S. Department of Energy, Office of Industrial Technologies, Washington, D.C., describes the subject invention. This document was actually published at an unknown later date.
Stoecker, W. F. et al. in xe2x80x9cEnergy Considerations in Hot-Gas Defrosting of Industrial Refrigeration Coils,xe2x80x9d Report No. 2796, ASHRAE Project RP-193, in Energy Consideration in Hot-Gas Defrosting of Industrial Refrigeration Coils,xe2x80x9d presented at the 1983 Annual Meeting of ASHRAE and published in the ASHRAE Transactions: Technical and Symposium Papers for that conference, (1983), pp. 549-573, describes the use of hot-gas defrosting of industrial refrigeration coils.
There are a number of secondary parameters that can be measured to determine when a coil is frosted, as evidenced by the teachings of the patents and publications that describe the demand defrost technologies cited above. None of these devices measure the primary parameter of interest for a refrigeration system: the rate of heat transfer from the air to the refrigerant.
In summary, although a need is recognized for a demand defrost controller as evidenced by previous efforts, no suitable equipment is available. In particular, the background art does not teach the concept of incorporating a heat flux sensor into a frost sensor for measurement of the rate of heat transfer from the air to the refrigerant.
A purpose of the invention is to provide a signal which can be use to initiate and terminate defrost cycles in industrial and commercial refrigeration systems in such a way as to reduce the overall energy consumed by the complete refrigeration system. Another purpose of the invention is to optimize the operation of evaporative heat exchangers and refrigeration systems.
A preferred embodiment of the invention is a frost sensor that can directly measure the total thermal resistance to heat flow from the air to the evaporative cooler fins. The frost sensor measures the reduction in heat flow due to the added thermal resistance of the ice (reduced conduction) as well as the reduction in heat flow due to the blockage of airflow (reduced convection) from excessive ice formation. This frost sensor triggers a defrost cycle when needed, instead of on a timed interval. Temperature sensors are incorporated into the frost sensor and are monitored to determine when to terminate the defrost cycle. This decreases the number of defrost cycles and the length of each defrost cycle to which a refrigeration system is subjected. A reduction in the total time a refrigeration system is defrosting increases the overall efficiency of the refrigeration process and thereby saves energy.
This disclosed demand defrost system also increases the cooling capacity of the refrigeration system by maximizing the cooling time compared to the defrost time. As a result, the refrigerated product is subject to fewer and shorter temperature excursions, thereby maintaining a higher level of product quality and reducing health risks for food products. Another advantage of a preferred embodiment of the invention is that it can be configured to trigger an alarm for any failure in the refrigeration system that limits the system""s ability to cool product. The alarm threshold can be set to trigger prior to the warming of the refrigerated space so that no product is lost. Because this frost sensor measures heat flux, any sudden decline in heat flux indicates that the refrigeration system has failed. These failure modes include air blockage, fan failure, loss of refrigerant, compressor failure, etc.
Another advantage of the invention is that it can be used to monitor the build-up of dirt on the evaporative coil over time. A preferred embodiment of the invention can maintain a record of the heat flux immediately after defrost, and compare the measurement after the most recent defrost cycle to identify the slow loss of efficiency due to the build-up of dirt. A preferred embodiment of the invention can then provide an alarm that alerts the user to an efficiency decline of the cooling coil.
One object of the invention is to increase the efficiency of refrigeration systems. Another object to allow a defrost cycle to be initiated when needed, instead of on a fixed time interval. Yet another object of the invention is to allow termination of a defrost cycle when frosting has been reduced adequately. A further object of the invention is to reduce the number of defrost cycles and the length of each cycle. Another object of the invention is to conserve energy. Yet another object of the invention is to increase the cooling capacity of refrigeration systems. A further object of the invention is to allow a higher level of refrigerated product quality to be maintained. Another object of the invention is to trigger an alarm when either equipment failure or more permanent insulating accumulations than frost (e.g., dirt buildups) occur. Yet another object of the invention is to measure insulating accumulations with a heat flux sensor that accumulates the deposits at about the same rate as adjacent fins.
The invention is an apparatus and method for controlling the operation of a refrigeration system that comprises an evaporative heat exchanger that transfers heat from air being cooled to a refrigerant. The disclosed invention measures the heat flux or total thermal conductivity of the thermal path from the air to the refrigerant, recognizes the reduction in heat flux or total thermal conductivity due to the thermal insulation effect of the frost and due to the loss of airflow from excessive ice formation, and controls the defrosting of the system accordingly.
In a preferred embodiment, the invention includes a heat flux sensor that presents a side to the airflow that has a temperature that is within a few degrees of the temperature of the adjacent portion of the fin upon which the heat flux sensor is installed, which fin is being used to transfer heat from the airflow to the refrigerant.
A preferred embodiment of the invention is a frost sensor for mounting on an evaporative heat exchanger that is exposed to an airflow, the frost sensor comprising: a printed circuit board comprising an electrical circuit; a first temperature sensor that is mounted on the printed circuit board and that is electrically connected to the electrical circuit; a copper clip that is preferably fabricated from full-hard spring copper; a heat flux sensor that is mounted on said copper clip and that is electrically connected to the electrical circuit; a spring clip that is operative to hold the heat flux sensor or said copper clip and the first temperature sensor against a first cooling fin of the evaporative heat exchanger, and to hold a part of the printed circuit board against the first cooling fin on the side of the cooling fin opposite the side against which the heat flux sensor or copper clip is being held; a second temperature sensor that is mounted on the printed circuit board in such a manner that the airflow impinges on the second temperature sensor before it impinges on the first cooling fin; and an electrical connector that is mounted on the printed circuit board and electrically connected to the electrical circuit. In a preferred embodiment, the disclosed frost sensor""s heat flux sensor is mounted on the copper clip by means of an adhesive (e.g., a thin layer of epoxy) and the copper clip is held against the first cooling fin.
In another preferred embodiment, the disclosed frost sensor further comprises a third temperature sensor that is in thermal contact with the evaporative coil at a location where ice tends to melt the slowest and that is thermally insulated from said airflow.
Another preferred embodiment of the invention is a defrost control system that comprises the disclosed frost sensor. The defrost control system comprises a signal processor (e.g., a microcontroller) and has the capability of controlling the temperature in the refrigerated zone, eliminating the need to interface with various commercially-available thermostatic controls. The defrost control system includes the disclosed defrost sensor and convention control components, which conventional components are known in the art and disclosed in the patents incorporated by reference herein above.
Another preferred embodiment of the invention is a refrigeration system that comprises the disclosed frost sensor. In refrigeration systems using advance control systems, the disclosed frost sensor can be used in combination with a processor that operates in accordance with the defrost algorithm disclosed herein. The refrigeration system includes the disclosed defrost sensor and conventional refrigeration components, which conventional components are known in the art and disclosed in the patents incorporated by reference herein above.
In another preferred embodiment, the invention is a device for sensing frost on a cooling fin of an evaporative heat exchanger that is subjected to an airflow, the device comprising: a heat flux sensor that is in thermal contact with and located on one side of the fin; a thermal insulator (e.g., a portion of a printed circuit board) that is in thermal contact with the fin and located on the other side of the fin, opposite the location of the heat flux sensor; a first temperature sensor that is in contact with the airflow before the airflow is in contact with the fin; and a second temperature sensor that is in thermal contact with the fin and that is thermally insulated from the airflow.
In yet another preferred embodiment of the invention, the disclosed device further comprises a third temperature sensor that is in thermal contact with the evaporative heat exchanger at a location where ice tends to melt the slowest and that is thermally insulated from the airflow. In a preferred embodiment, the device""s heat flux sensor is a differential thermopile and the thermal insulator is at least a portion of a printed circuit board.
A further preferred embodiment of the invention is a sensor for characterizing the heat-transfer effectiveness of an evaporative coil that comprises a plurality of cooling fins that are subject to insulating accumulations, the sensor comprising: means (e.g., a heat flux sensor) for measuring the heat flow from an airflow to a first cooling fin that produces a first signal; means (e.g., a first temperature sensor) for measuring the temperature of the first cooling fin that produces a second signal; and means (e.g., a second temperature sensor) for measuring the temperature of the airflow that produces a third signal; wherein the three signals are used to quantify the total thermal conductivity of the accumulations. In a preferred embodiment, the third signal is used to determine when the coil has been adequately defrosted during a defrost cycle. In another preferred embodiment, the sensor also comprises means (e.g., a computer) for scheduling the defrost cycle for the evaporative coil.
In another preferred embodiment, the disclosed sensor further comprises means (e.g., a third temperature sensor) for measuring the temperature of a second cooling fin that produces a fourth signal, the second cooling fin being located where ice tends to melt more slowly than it does on the first cooling fin during a defrost cycle. Preferably, the means for measuring the temperature of a second cooling fin is installed in a region of the evaporative coil where ice (frost) tends to melt the slowest. In a preferred embodiment, the fourth signal is used to determine when the coil has been adequately defrosted during a defrost cycle.
Another preferred embodiment of the invention is a device for sensing frost on a cooling fin of an evaporative heat exchanger, the device comprising a heat flux sensor that is in thermal contact with the fin. The heat flux sensor produces signals that are used to control the defrosting of the evaporative heat exchanger.
The invention is also a method of using or operating the disclosed apparatus. In a preferred embodiment, the invention is a method for defrosting a refrigeration system that includes an evaporative heat exchanger having a fin that is exposed to an airflow and that is subject to an insulating accumulation, the method comprising: measuring the heat flux from the airflow into the fin, the temperature of the fin and the temperature of the airflow; calculating the total thermal conductivity or total thermal resistance value of the insulating accumulation (should an accumulation exist); and initiating a defrost cycle when the total thermal conductivity or total thermal resistance value reaches a predetermined setpoint. Preferably, the disclosed method further comprises terminating said defrost cycle when the temperature of the fin reaches a target temperature.
In a preferred embodiment, the disclosed method also comprises comparing heat flux data collected at a previous time with heat flux data collected at a subsequent time; and initiating an alarm procedure if the comparison indicates that the measured heat flux is trending downward.
Yet another embodiment of the invention is a technique for operating a refrigeration system that includes an evaporative heat exchanger having a fin that is exposed to an airflow and that is subject to an insulating accumulation, the technique comprising: measuring the heat flux from the airflow into the fin, the temperature of the fin and the temperature of the airflow; determining whether the heat flux is below an expected value; if the heat flux is not below the expected value, calculating the total thermal conductivity or total thermal resistance value of the insulating accumulation and applying a setpoint adjustment factor; resetting the system timers; determining whether the system is in an auto defrost mode; if the system is in an auto defrost mode, determining whether the heat flux or the total thermal conductivity or total thermal resistance value has reached a setpoint; if the system is not in an auto defrost mode, determining whether the time until defrost has expired; and if either the heat flux or the total thermal conductivity or total thermal resistance value has reached a setpoint or the time until defrost has expired, initiating a defrost cycle.
In another embodiment, the disclosed technique further comprises: if the heat flux is below the expected value, determining if the heat flux is downward trending; if the heat flux is downward trending, concluding that an evaporator dirt accumulation is indicated, sending an evaporator dirt accumulation alarm message and setting the system to a timer mode; and if the heat flux is not downward trending, concluding that an equipment failure is indicated, sending an equipment failure alarm message and setting the system to a time mode.
A further preferred embodiment of the technique further comprises: if a defrost cycle has been initiated and the system is in an auto defrost mode, determining whether the fin temperature has reached a fin temperature target; if the fin temperature has reached the fin temperature target, comparing the actual defrost time to the target defrost time; if the actual defrost time is less than the target defrost time, decreasing the setpoint adjust factor; if the actual defrost time is greater than the target defrost time, increasing the setpoint adjust factor; and if the system is not in the auto defrost mode, determining whether the time until defrost has expired.
Yet another preferred embodiment of the invention is a process for control of frost in a system that transfers heat from air to a refrigerant along a thermal path, the process comprising: measuring the thermal conductivity of the thermal path from the air to the refrigerant; recognizing a reduction in thermal conductivity due to the thermal insulation effect of the frost and due to the loss of airflow from excessive ice formation; and controlling the defrosting of the system.
Further aspects of the invention will become apparent from consideration of the drawings and the ensuing description of preferred embodiments of the invention. A person skilled in the art will realize that other embodiments of the invention are possible and that the details of the invention can be modified in a number of respects, all without departing from the inventive concept. Thus, the following drawings and description are to be regarded as illustrative in nature and not restrictive.